When an electron-positron pair annihilates, the energy released is about

Electron and positron are identical in all respect except that charges on them are opposite when an electron and a positron meet together, they completely annihilate to each other and two photons are produced. 

${\mathrm{e}}^{+}+{\mathrm{e}}^{-}\to 2\mathrm{\gamma }$

The rest mass energy of each particle, electron and positron is 0.511 MeV. Hence, 2 x 0.511 MeV energy of photon is converted to rest mass energy of two particles. 
Total energy is

$$\begin{gathered} \mathrm{E}\text{'}=2 \mathrm{x} 0.511 \mathrm{x} {10}^6 \mathrm{x} 1.6 \mathrm{x} {10}^{-19} \\ \mathrm{E}\text{'}=1.6 \mathrm{x} {10}^{-13}\mathrm{J} \end{gathered}$$